I'm glad someone else has seen the movie/read the book!!!
I just moved yesterday to another state about 5 hours away. I put Liv in her carrier about an hour or so before I left so she could acclimate (and to prevent her from escaping while we were moving the last bit of boxes). I did give her food and water early in the morning, as well as gabapentin. A couple hours later she vomited everything up and then had diarrhea so just something to watch for. She's taken gabapentin before (she has to take a dose before vet visits) and she's never had this reaction before. Once we got going she was fine for the most part. She would yowl and hiss occasionally if we hit a bump in the road, but overall I thought it went well. I would say total she was in the carrier about 7 hours. I did give her food and water as soon as we got to my new place and she did drink quite a bit of water after she settled down. If I had to do it over again, I'd probably restrict food intake before the move (definitely not before the gabapentin) and maybe take a break in between for water (we just drove straight through). I've read some cats won't eat or drink when in the process of moving and I honestly don't think she would have either, but it's something to consider. Also, Liv is a very nervous cat so I'm sure that also impacted her behavior.
If you look at the mechanism of how the drug works it makes sense that it could be used to decrease anxiety. It's basically reducing activity in the brain which is why it controls seizures. Psychiatrists also prescribe this medication off label for anxiety in humans as well. I think this is why it also works for cats.I don't understand why vets prescribe gabapentin for car rides. It is for controlling seizures in humans with epilepsy (marketed as Neurontin) and not intended to be an antianxiety drug.